The present invention relates generally to reference receivers and more specifically to discrete filter-less optical reference receiver and output amplifier having a system frequency response curve predominantly established by a sampling circuit.
A reference receiver is a measurement instrument that is bounded in the frequency domain by defined tolerance limits. Ideally, the frequency response curve should be Gaussian but such a response can be difficult to achieve, especially at high data rates, such as 10 Gb/s and beyond. In the telecommunications industry, the typical reference receiver has a has a system frequency response curve that matches a Bessel-Thompson filter response. The predominant frequency response of previous reference receivers has traditionally been established by discrete reference receiver filters at the input of the receiver having frequency responses that match a 4.sup.th or 5.sup.th order Bessel-Thompson frequency response.
The telecommunications industry is continually establishing standards for the ever increasing data rates used in the industry. Most telecommunications and data communications industry standards define acceptable scalar frequency response characteristics that test equipment must have when performing eye-pattern conformance testing of fiber based optical signals. The industry standards define the scalar frequency response as having a Bessel-Thompson shape (near Gaussian) with the -3 dB electrical power response rolloff point at 3/4 of the bit rate. For example, a reference receiver for a 2.488 Gbps SONET signal would have a nominal -3 dB point at 1.87 GHz. FIG. 1 shows a graphical representation of the Bessel-Thompson shape scalar frequency response for a generic reference receiver. The frequency response has upper and lower tolerance limits that the reference receiver must meet for telecommunications and data systems.
As new standards are being proposed, the telecommunications standards committees approach test equipment manufacturers to solicit proposals for establishing the upper and lower tolerance limits for the proposed standards. Tolerance limits proposed by test equipment manufactures are based on the reference receiver design and the components used in the design. The data profile in FIG. 1 shows aberrations in the data that may be caused by a number of factors. A significant cause of data aberrations is commonly the discrete reference receiver filter.
Referring to FIG. 2, there is shown a typical reference receiver 10 designed for measuring optical signals by the telecommunications industry. The reference receiver has an optical-to-electrical (O/E) converter 12 that receives the optical signal under test. Typically the O/E converter is a photodiode 14 that converts the optical signal to a corresponding electrical signal. A transmission line couples the electrical signal to a first switch 16 that is connected via a second switch 18 to a series of discrete reference receiver filters 20, 22. Transmission lines are also used to connect the switches and filters together. The O/E converter 12 includes a termination resistor or resistors 24 that reverse terminates the converter 12 in the characteristic impedance of the transmission line. The reference receiver filters 20, 22 are selectively switched into the reference receiver depending on the data rate of the optical input signal. For example, the discrete reference receiver filters may have a Bessel-Thompson shape scaler frequency response for 622 Mbps and 9.953 Gbps data rates. The filtered electrical signal output of the respective discrete reference receiver filters 20, 22 are selectively coupled via a third switch 26 to a fourth switch 28 that also receives the unfiltered electrical signal from the O/E converter 12 via a separate transmission line. The fourth switch 28 selectively couples the filtered electrical signal and the unfiltered electrical signal to a sampling circuit 30. The sampling circuit 30 includes a termination resistor 32 that terminates the transmission line in its characteristic impedance. Generally, the transmission lines have a 50 ohm characteristic impedance requiring 50 ohm termination resistors. The sampling circuit 30 includes two series connected diodes 34, 36 having a common node 38 receiving the filtered and unfiltered electrical signals. Opposing negative and positive strobe pulses are applied to the opposing sides 40, 42 of the diodes for gating the diodes 34, 36 into conduction. Positive and negative bias voltages are applied to the respective diodes via resistors 44, 46 to bias the diodes 34, 36 for selected turn on and turn off times.
A number of drawbacks are associated with current reference receiver designs. One major drawback is the generation of reflections from the discrete electrical filters requiring reverse termination of the optical-to-electrical converter. The reverse termination resistor absorbs a substantial portion of these reflected signals, for example 95% of the reflected signal, and provides a good temporal response in the presence of discrete filters. If the discrete filter produces a 10% reflection, then there is a 1/2% reflection that is coupled into the sampling circuit. Such reflections cause aberrations in the sampled signal as represented by the aberrations in the data in FIG. 1. These types of aberrations are taken into account by test equipment manufactures in making recommendations for the tolerance limits for reference receiver standards. Another drawback to current reference receiver designs is that the reverse termination resistor generally tends to flatten the frequency response of the system and produce a much steeper roll-off than a non-reverse terminated system. The result is further deviation from the ideal Gaussian response and renders the system with more aberrations when using it for non-filter applications when all-out bandwidth is desired. A discrete electrical filter in the high frequency path also increases the possibility of group delay distortion caused by the filter itself.
FIG. 3 is a schematic representation of a prior art output amplifier circuit 50 associated with a prior art reference receiver 10. Operational amplifiers 52, 54 have their respective non-inverting input terminals connected to bias voltages +V.sub.bb and -V.sub.bb. The inverting input terminals of the operational amplifiers 52, 54 are set to the respective bias voltage levels of the non-inverting input terminals by feedback through resistors 56, 58. The bias voltages on the inverting input terminals are coupled to the sampling circuit 30 as the biasing voltages for the sampling diodes 34 and 36. An offset voltage may be applied to the sampling diodes 34 and 36 that shifts the bias voltages levels in common mode. The feedback resistors 56, 58 in the operational amplifier circuits 52, 54 have a high ohmic value in the range of 100 Megohms to reduce the amplifiers noise for generating a cleaner output signal. The output signals from the operational amplifiers 52, 54 are summed together at the inverting input terminal of a summing amplifier 60. The output signal from the summing amplifier 60 is digitized and further processed to produce a display on a display device. A zero volt input to the sampling circuit 30 with no bias offset causes the relative bias level of each diode to be equal. The positive and negative strobe pulses drive the sampling diodes 34 and 36 into conduction with the resulting sampling charge from each diode being balanced. The resulting capacitor charge on each of the operational amplifiers 52, 54 generates respective integrated voltages at the output of the amplifiers that cancel each other out. Any non-zero voltage input unbalances the instantaneous total bias between one diode and the other resulting in a measurable difference in the integrated voltages of the operational amplifiers 52, 54.
Current reference receivers with this type of amplifier strobe the sampling circuit 30 in a range of 40 to 50 Ksamples/second. The respective strobe signals to the sampling circuit are rectified by the sampling diodes 34, 36 resulting in a series of charge impulses into the amplifiers 52, 54. The charge pulses are integrated to produce a DC current through each of the 100 Megohm feedback resistors 56, 58 in the operational amplifiers 52, 54 resulting in a DC voltage at the respective amplifiers 52, 54 outputs. A 40 Ksamples/sec strobe could, for instance, produce a 50 nanoamp DC current through the respective feedback resistors 56, 58, which produces an output voltage level of 5 volts. Increasing the strobe rate increases the integrated DC currents coupled through the operational amplifiers 52, 54. At a certain rate, the DC currents through the feedback resistors 56, 58 overdrives the operational amplifiers 52, 54 causing distortion of the output signals.
What is needed is a time domain reference receiver sampling system for optical signals that does not have the drawbacks of conventional reference receivers having discrete reference receiver filters. The reference receiver sampling system should have a optical-to-electrical converter with a high impedance source for generating an electrical signal having an optimum temporal response more closely resembling the actual optical signal and configured to effectively utilize the current output of the high impedance source. The reference receiver sampling system should also have a sampling circuit that is strobed at a substantially higher sampling rate than conventional reference receiver sampling systems. The sampling circuit should have an output amplifier circuit that can receive output signals at the conventional and higher strobe rate without generating a distorted output signal.